The interaction of thrombin with platelets will be investigated by further characterization of binding and by attempts to isolate the receptor. Characterization will involve pursuit of 2 recent observations. 1) There is a time-dependent transition from a rapidly dissociating thrombin-platelet complex to a more slowly dissociating complex, and the transition is observed only with active thrombin; experiments are proposed to determine whether the transition involves the active site of thrombin or is a consequence of platelet stimulation. 2) Chymotrypsin treatment of platelets alters, but does not decrease, the response of platelets to thrombin (but not to other stimuli) with characteristics that suggest separation of the binding step from the signal generation step; further attempts to define the effect of chymotrypsin on platelet surface proteins are proposed. Attempts at isolation of the receptor will involve the use of hetero-bifunctional, photoactivatable cross-linking reagents and affinity chromatography using isolublized thrombin.